This invention relates to the application of toner to an electrostatic image to create a toner image. More specifically, this invention relates to an improvement in the development of an electrostatic image using a two component developer made up of charged toner particles and oppositely charged hard magnetic carrier particles.
U.S. Pat. No. 4,546,060, Miskinis et al issued Oct. 8, 1985, discloses a method of developing electrostatic images using developer including a "hard" magnetic carrier having a coercivity of at least 300 gauss when magnetically saturated and exhibiting an induced magnetic moment of at least 20 emu/gm of carrier when in an applied field of 1000 gauss. A preferred embodiment of this carrier having much higher coercivity, in the neighborhood of 2000 gauss, with a higher magnetic moment, about 55 emu/gm, is commercially used to provide the highest quality of electrostatic image development presently available. In this method, developer made up of such hard carrier particles and oppositely charged toner particles is moved at the same speed and direction as the image by high speed rotation of a magnetic core within a shell or sleeve on which the developer moves. Rapid pole transitions on the shell are mechanically resisted by the carrier because of its high coercivity. "Strings" or "chains" of the carrier rapidly flip on the shell to move with the toner on the shell through developing relation with the electrostatic image. See, also, U.S. Pat. No. 4,473,029, Fritz et al, and U.S. Pat. No. 4,531,832, Kroll et al. These two patents and the Miskinis patent are hereby incorporated by reference herein.
The rapid pole transitions, for example, as many as 400 per second on the shell surface when at a core speed of 1500 rpm, create great vigorousness in the developer as it moves through the development zone. This vigorousness constantly recirculates the toner to the shell surface and then back to the outside of the nap to provide it with its desired charge for development. It also continually feeds fresh toner to the image. This system provides high density, high quality images at high development speed.
The direct interaction of the developer nap with the image member causes the developer to roll back toward the input side of the development zone. This rollback broadens the contact between the developer and the image member and improves the development completion of the system. However, it also has a tendency to separate the carrier somewhat from the magnetic fields of the core with the result that some carrier particles are picked up in the image itself. This carry out of carrier by the image is a well known phenomenon in two component developing that occurs, to some extent, in virtually all two component, contact systems. Unfortunately, carrier particles have a bad effect, not only on the image itself, but on the rest of the system. They are generally of the wrong charge to transfer but will prevent toner particles from transferring around them, leaving white spots in the image. If they do transfer, they do not necessarily fuse well. These and other problems associated with carrier being carried away by the image are well documented.
A number of approaches have been devised for removing carrier from an image as or after the image member leaves the development station. For example, most image forming apparatus with this problem include a magnetic scavenger which attracts carrier magnetically from the image, either as part of the toning station or downstream of it. It is also known to attract the carrier electrically, since it is charged opposite to the toner. For example, U.S. Pat. No. 4,647,186, issued Mar. 3, 1987 to Armstrong et al, shows the use of a set of wires immediately after the development zone to which an AC potential is applied with a DC component attractive to carrier having a charge opposite that of the toner.
A large number of references suggest that a high voltage AC bias can be impressed upon the electrical field between a magnetic brush and an electrostatic image to increase the development completion of the electrostatic image. See, for example, U.S. Pat. Nos. 4,933,254, Hosoi et al, issued Jun. 12, 1990; 4,873,551, Tajima et al, issued Oct. 10, 1989; 4,838,200, Hosoi et al, issued Jun. 13, 1989; and 4,565,438, Folkins, issued Jan. 21, 1986. Prior commercial applications of the Miskinis method of development have not used an alternating current component to the development electrical field because the vigorousness of the developer movement was considered more than adequate for development completion. Thus, all present commercial applications of this particular system use only a direct current field.